The twin Voyager spacecraft, over the course of a dozen years,
drew back the curtain on nearly half of the solar system. From
launch in 1977 through the spectacular parting shots of Neptune
at the outer reaches of the solar system in 1989, this pair of
spacecraft explored four planets -- Jupiter, Saturn, Uranus and
Neptune -- as well as dozens of moons, and the rings and
magnetic environments of those planetary systems.

The Voyagers were designed to take advantage of a rare geometric
arrangement of the outer planets that occurs only once every 176
years. This configuration allows a single spacecraft to swing
by all four gas giants without the need for large onboard
propulsion systems; the flyby of each planet both accelerates
the spacecraft and bends its flight path. Without these gravity
assists, the flight time to Neptune would have been 30 years.

The second of the two Voyager spacecraft, Voyager 2, was
launched first, on 20 August 1977. It was followed on 5
September 1977 by Voyager 1, which was put on a faster, shorter
trajectory to Jupiter. Both launches took place from the Cape
Canaveral Air Force Station in Florida.

Eighteen months after launch, Voyager 1 reached Jupiter, 650
million kilometers away. The spacecraft made its closest
approach on 5 March 1979, while Voyager 2 followed on 9 July
of the same year. Images streamed back from the pair of
spacecraft showing the complex, swirling turbulence of
Jupiter's atmosphere in exquisite detail. A giant storm, three
times the size of Earth, raged in Jupiter's upper atmosphere,
surrounded by rippling currents that rotated about it.
Voyager 1 found nine active volcanoes erupting on Io, the
innermost of Jupiter's four major moons. Four months later,
Voyager 2 found that eight of the nine volcanoes were still
active. A thin, dusty ring was also discovered around Jupiter,
forcing revision of theories about origins and mechanics of
planetary ring systems.

At Saturn, both Voyagers took high-resolution images to help
determine ring composition and dynamics. The Voyager 1
encounter took place in November 1980 and the Voyager 2
encounter was in August 1981. Voyager 1 was targeted to fly
close to Saturn's largest moon Titan. This resulted in a
south polar passage of Saturn, which redirected the spacecraft
northward of the ecliptic.

Voyager 2 continued on to Uranus where ten new moons were
discovered in the Uranus system. The planet's magnetic
field was found to be significantly offset from the planet's
axis of rotation.

In August 1989, Voyager 2 flew past Neptune. Because Neptune
receives so little sunlight, many scientists had expected to
see a placid, featureless planet. Instead, Voyager showed a
dynamic atmosphere with winds blowing westward, opposite the
direction of rotation, at speeds faster than the winds of any
other planet. Neptune revealed its Great Dark Spot, a storm
system that resembled Jupiter's Great Red Spot, and a smaller,
eastwardly moving cloud, called 'scooter', which went around
the planet about every 16 hours. The blue planet was circled
by diffuse, dusty rings; six new moons were discovered.

Voyager 2 passed over the north polar region on Neptune,
using the planet's gravity to redirect the trajectory for a
final encounter -- with Neptune's largest moon Triton. It then
departed the solar system southward of the ecliptic.

At about the same time as Voyager 2 was encountering Neptune,
Voyager 1, continuing its journey to the edge of the solar
system on the north side of the ecliptic, turned its cameras
back to look at the planets and take one last parting shot.
Voyager 1's 'family portrait' illustrates the vastness of the
solar system and the huge expanses of emptiness within which
the outer planets lie.

Both Voyagers are now headed for the outer boundary of the
solar system, where the Sun becomes just one of many
contributors to the interstellar environment. That edge is
thought to be somewhere between 8 billion and 23 billion
kilometers from the Sun. Engineers are optimistic that the
Voyagers will still be transmitting data when that boundary is
encountered sometime in the first quarter of the twenty-first
century.

The spacecraft were assembled at and the mission was managed by
the Jet Propulsion Laboratory, Pasadena, CA. Early parts of
the mission have been described in more detail by
[MORRISON1982].

The launch vehicle for Voyager 1 was a Titan/Centaur. The
first stage Titan was powered by both solid and liquid fuel
engines. The Centaur stage, 20 meters long and 3 meters in
diameter burned a fuel combination of liquid hydrogen and
liquid oxygen. The Titan boosted the Voyager Centaur
combination into low Earth orbit, and the Centaur plus a
small solid fuel rocket provided the energy for Voyager 1 to
escape Earth orbit.

The Voyager 1 flyby of Jupiter took place on 5 March 1979 at
12:04:36 UTC with the spacecraft closest approach only 348890
kilometers from the center of Jupiter. Among the highlights
of the encounter were the discovery of a faint ring and one
new satellite. Satellite encounter information is given
below; 'UNK' denotes 'unknown' at time of this writing. The
Voyager 1 Jupiter encounter is described in more detail by
[STONE&LANE1979A].

During the period between Jupiter Encounter and Saturn
Encounter, Voyager 1 probed the interplanetary medium,
observed selected celestial targets, and conducted tests and
calibrations of its systems. Mission planners used the 16
months to develop and test activity sequences which would be
used during the Saturn Encounter.

The Voyager 1 flyby of Saturn took place on 12 November 1980
at 23:46 UTC with the spacecraft closest approach only 184300
kilometers from the center of Saturn. Among the highlights
of the encounter were the separate encounter with Titan,
discovery of intricate patterns within the ring system, and
observation of variations among the many moons of Saturn.
Closest approaches to some of the satellites were on the
dates and at the distances shown below. 'UNK' denotes
'unknown' at the time of this writing. The encounter is
described in more detail by [STONE&MINER1981].

After conclusion of the Saturn Encounter, Voyager 1 left the
ecliptic at an angle of about 30 degrees. Its scan platform
instruments were turned off, but some of the remaining
instruments (primarily fields and particles) continued to
monitor the environment in the outer solar system as the
spacecraft traveled outward toward the heliopause.

The launch vehicle for Voyager 2 was a Titan/Centaur. The
first stage Titan was powered by both solid and liquid fuel
engines. The Centaur stage, 20 meters long and 3 meters in
diameter burned a fuel combination of liquid hydrogen and
liquid oxygen. The Titan boosted the Voyager Centaur
combination into low Earth orbit, and the Centaur plus a
small solid fuel rocket provided the energy for Voyager 2 to
escape Earth orbit.

The Voyager 2 flyby of Jupiter took place on 9 July 1979 at
22:29 UTC. This was 18 weeks after the Voyager 1 Jupiter
Encounter and was at a closest approach distance of 721670
kilometers from the center of Jupiter. The Voyager 2
trajectory was chosen to complement that of Voyager 1,
including a much closer approach to Europa, probing southern
latitudes in Jupiter's atmosphere, and an extensive
investigation of Jupiter's magnetotail. Satellite encounter
information is given below; 'UNK' denotes 'unknown' at time
of this writing. The Voyager 2 Jupiter encounter is
described in more detail by [STONE&LANE1979B].

During the period between Jupiter Encounter and Saturn
Encounter, Voyager 2 probed the interplanetary medium,
observed selected celestial targets, and conducted tests and
calibrations of its systems. Mission planners used the 22
months to develop and test activity sequences which would be
used during the Saturn Encounter.

The Voyager 2 closest approach to Saturn was on 26 August
1981 at 03:24 UTC and at a distance of 161000 km from the
center of Saturn. The trajectory was chosen so that the
spacecraft could obtain a gravitational assist from Saturn
and continue on to Uranus; the timing was selected to provide
better views of several satellites than had been obtained
from Voyager 1. Design of science sequences was influenced
by Voyager 1 results. Satellite encounters were on the dates
and at the closest approach distances shown below; 'UNK'
denotes 'unknown' at the time of this writing. The
scan platform seized temporarily 110 minutes after Saturn
closest approach, causing the central computer to disable
further commands and resulting in loss of some data. When
commanded again three days later (at low rate), it moved as
instructed. A gyroscope calibration error between closest
approach and five hours later also caused loss of data.
Scan platform activities ended on 5 September 1981. This
encounter is described in more detail by [STONE&MINER1982].

During the period between Saturn Encounter and Uranus
Encounter, Voyager 2 probed the interplanetary medium,
observed selected celestial targets, and conducted tests and
calibrations of its systems. Mission planners used the 49
months to develop and test activity sequences which would be
used during the Uranus Encounter. Considerable attention was
paid to the scan platform capabilities, following its seizure
during the Saturn Encounter. Full scan platform operation
was restored before the end of 1981.

The Voyager 2 closest approach to Uranus was on 24 January
1986 at 17:59 UTC at a distance of 107000 km from the
center of Uranus. The trajectory was chosen so that the
spacecraft could obtain a gravitational assist from Uranus
and continue on to Neptune; NASA permission for the Neptune
Encounter was granted during the approach to Uranus. The
timing of the Uranus closest approach was selected to
provide a close approach to Miranda and to allow capture of
radio occultation data at the DSN tracking station in
Australia (southern declination of Uranus meant that
Australia was preferred for DSN tracking). Radio
occultation data were also collected using the 64-m
antenna at Parkes in Australia. Satellite encounters were
on the dates and at the closest approach distances shown
below. 'UNK' denotes 'unknown' at the time of this writing.
Satellite images were improved by implementation of image
motion compensation on the spacecraft. Reed-Solomon encoding
was used for the first time; real-time imaging data rates
were reduced by almost 70 percent. Ground antennas were
arrayed to increase receiving aperture. This encounter is
described in more detail by [STONE&MINER1986].

During the period between Uranus Encounter and Neptune
Encounter, Voyager 2 probed the interplanetary medium,
observed selected celestial targets, and conducted tests and
calibrations of its systems. Mission planners used the 39
months to develop and test activity sequences which would be
used during the Neptune Encounter.

The DSN used this time to add a 34-m tracking antenna at the
Madrid complex, to increase the diameter of their 64-m
antennas to 70 meters, and to make the 70-m systems more
efficient. A special microwave link was installed to permit
the Parkes radio telescope to be arrayed with the Canberra
DSN antenna in Australia.

The Voyager 2 closest approach to Neptune was on 25 August
1989 at 03:56 UTC at a distance of 29240 km from the
center of Neptune. The trajectory and timing were chosen so
that the spacecraft could obtain a gravitational assist from
Neptune and continue on for an encounter with Neptune's large
satellite Triton about five hours later (closest approach at
09:10 UTC). The timing was also selected so that radio
occultation data would be collected at the DSN tracking
station in Australia (southern declination of Neptune meant
that Australia was preferred for DSN tracking). Radio
occultation data were again collected with the Parkes antenna
and with a new 64-m antenna at Usuda in Japan. Satellite
encounters were on the dates and at the closest approach
distances shown below. 'UNK' denotes 'unknown' at the
time of this writing. Data rates were increased over those
at Uranus by including the Very Large Array (VLA) in New
Mexico for receiving and by taking advantage of DSN upgrades
made over the previous three years. This encounter is
described in more detail by [STONE&MINER1989].

After conclusion of the Neptune Encounter, Voyager 2 left the
ecliptic at an angle of about -30 degrees. Its scan platform
instruments were turned off, but some of the remaining
instruments (primarily fields and particles) continued to
monitor the environment in the outer solar system as the
spacecraft traveled outward toward the heliopause. During
the Shoemaker-Levy 9 impact with Jupiter in July 1994, the
ultraviolet spectrometer was trained on Jupiter and radio
signals were recorded; but no emissions from the impact were
detected.

Voyager's primary objective was exploration of the two giant
planets, Jupiter and Saturn, their magnetospheres, and their
satellites. Major emphasis was placed on studying the
satellites, many of which are planet-sized worlds, in as much
detail as possible. The study of Titan, the only satellite in
the solar system known to have an extensive atmosphere, was
nearly as high a priority as studies of Saturn itself
[MORRISON1982]. After the successful Voyager 1 encounter with
Titan, it was decided to expand the Voyager objectives to
include at least Uranus; Uranus and Neptune could both be
reached by proper reprogramming of the Voyager 2 trajectory.
Comparative studies then could include the four largest planets
in the solar system.

Eleven investigations were approved for the Voyager mission.
Investigation names and Principal Investigators, or Team
Leaders in the cases of ISS and RSS, are shown in the table
below; the trailing 'S' stands for 'subsystem' in most
acronyms.

Broadly stated, the science goals of the mission were: high
resolution imaging of the gas planets and inference of
atmospheric dynamics; high resolution imaging of satellites and
inference of geologic processes; spectral measurements of
atmospheres and satellite surfaces, inference of compositions,
and inference of thermal properties and structure;
identification and study of aerosols and surface physical
structure using polarized light; occultation measurement of
atmospheric thermal, ionospheric charged particle, and ring
structure; and measurement of magnetic fields and particle
environments and inference of Sun-planet-satellite
interactions, magnetospheric structure, and mechanisms within
each planetary system for generating the observed fields.

The largest planet in the solar system, Jupiter is composed
mainly of hydrogen and helium, with small amounts of methane,
ammonia, water vapor, traces of other compounds and a core of
melted rock and ice. One of the objectives of Voyager was to
quantify the composition of the atmospheres of Jupiter and
the other giant planets.

Colorful latitudinal bands, atmospheric clouds, and storms
characterize Jupiter's dynamic atmosphere. By taking a
series of images, Voyager could show the time variability of
the atmosphere. The Great Red Spot was revealed as a complex
storm moving in a counterclockwise direction. An array of
other smaller storms and eddies were found throughout the
banded clouds.

Jupiter is now known to possess 16 moons. An objective of
the Voyager mission was to search for new moons and to obtain
high resolution quantitative measurements on those that had
been discovered earlier. Active volcanism on the satellite
Io was easily the most surprising discovery at Jupiter. It
was the first time active volcanoes had been seen on another
body in the solar system. Together, the Voyagers observed
the eruption of nine volcanoes on Io, and there is evidence
that other eruptions occurred between the Voyager encounters.

Although interpretations vary, the cratered surfaces of the
terrestrial planets (and the Moon) are believed to contain
the record of small body populations in the inner solar
system from as far back as 4 billion years ago. One of the
objectives of the Voyager mission was to obtain similar
cratering data from satellites in the outer solar system.
Impact craters on Io have been obliterated by that satellite's
volcanism. Rather than craters, Europa was distinguished by
a large number of intersecting linear features with almost no
topographic relief. There is a possibility that Europa is
internally active due to tidal heating at a level one-tenth
or less than that of Io and that the crust is very thin (less
than 30 kilometers). Ganymede has two distinct types of
terrain -- cratered and grooved -- suggesting that its entire
icy crust has been under tension from global tectonic
processes. Callisto has a very old, heavily cratered crust
showing remnant rings of enormous impact craters. The
largest craters have apparently been erased by the flow of
the icy crust over geologic time. Almost no topographic
relief is apparent in the ghost remnants of the immense
impact basins, identifiable only by their light color and the
surrounding subdued rings of concentric ridges.

Indirect evidence from Pioneer 10/11 suggested the presence
of a thin ring around Jupiter. One of the objectives of the
Voyager mission was to search more systematically for such a
ring, and to quantify both the number-density and the size
distribution of particles within rings in the outer solar
system. A faint, dusty ring of material was found around
Jupiter. Its outer edge is 129,000 kilometers from the
center of the planet, and it extends inward about 30,000
kilometers.

Two new, small satellites, Adrastea and Metis, were found
orbiting just outside the ring. A third new satellite,
Thebe, was discovered between the orbits of Amalthea and Io.

Jupiter's rings and moons exist within an intense radiation
belt of electrons and ions trapped in the planet's magnetic
field. These particles and fields comprise the jovian
magnetosphere, or magnetic environment, which extends three
to seven million kilometers toward the Sun, and stretches in
a windsock shape at least as far as Saturn's orbit -- a
distance of 750 million kilometers (460 million miles).

As the magnetosphere rotates with Jupiter, it sweeps past Io
and strips away about 1,000 kilograms (one ton) of material
per second. The material forms a torus, a doughnut-shaped
cloud of ions that glow in the ultraviolet. The heavy ions
in the torus migrate outward, and their pressure inflates the
jovian magnetosphere to more than twice its expected size.
Some of the more energetic sulfur and oxygen ions fall along
the magnetic field into the planet's atmosphere, resulting in
auroras.

A major objective of the Voyager mission was to determine in
which ways the gas giants are the same and in which ways they
are different. Saturn, like Jupiter, is mostly hydrogen and
helium. Its hazy yellow hue has broad atmospheric banding
similar to (but much fainter than) that found on Jupiter. It
also has a complex ring system, the details of which were
sketchy before Voyager, but which represented an important
objective in themselves.

It is thought that the rings formed from one or more moons
that were shattered by impacts of comets and meteoroids. The
resulting material, ranging in size from dust to house-sized
particles, has accumulated in a broad plane in which both the
shape and density vary in ways which depend intricately on
gravitational interactions with satellites. This is most
obviously demonstrated by the relationship between the F-ring
and two small moons that 'shepherd' the ring material. The
variation in the separation of the moons from the ring may
explain the ring's kinked appearance. Shepherding moons were
also found by Voyager 2 at Uranus. Very diffuse rings and
'spokes' (neither detected from Earth) were also found by
Voyager.

Winds blow at extremely high speeds on Saturn -- up to 1,800
kilometers per hour. Their primarily easterly direction
indicates that the winds are not confined to the top cloud
layer but must extend at least 2,000 kilometers downward into
the atmosphere.

Saturn has 18 known satellites ranging from Phoebe, a small
moon that travels in a retrograde orbit and is probably a
captured asteroid, to Titan, the planet-sized moon with
an atmosphere that had been detected from Earth before
Voyager. A major objective of Voyager was to
investigate these satellites and, in particular, to learn a
great deal more about Titan. Titan's surface temperature and
pressure were found to be 94 K and 1.6 atmospheres.
Photochemistry converts some atmospheric methane to other
organic molecules, such as ethane, that may accumulate in
lakes or oceans. Other more complex hydrocarbons form the
haze particles that eventually fall to the surface, coating
it with a thick layer of organic matter. The chemistry in
Titan's atmosphere may resemble that which occurred on Earth
before life evolved.

The most active surface of any moon seen in the Saturn system
was that of Enceladus. The bright surface of this moon,
marked by faults and valleys, showed evidence of tectonically
induced change. Voyager 1 found that the surface of Mimas is
dominated by a crater so large that the impact nearly broke
the satellite apart.

Saturn's magnetic field is weaker than Jupiter's, extending
only one or two million kilometers. The axis of the field is
almost perfectly aligned with Saturn's rotation axis.

Uranus is distinguished by the fact that it is tipped on its
side. This unusual orientation is thought to be the result
of a collision with a planet-sized body early in the solar
system's history. Clues to this event, as well as more basic
data about this planet (which has polar regions exposed to
sunlight or hidden in darkness for long periods) were
important Voyager objectives. At about the time of Voyager's
launch, observations from Earth showed that Uranus was
circled by rings -- not bright and wide, as was the case
for Saturn, but extremely narrow and very dark.

Voyager 2 found that one of the most striking influences of
the orientation of the rotation axis is its effect on the
tail of the magnetic field, which is itself tilted 60 degrees
from the planet's axis of rotation. The magnetotail was
shown to be twisted by the planet's rotation into a long
corkscrew shape behind Uranus.

The existence of a magnetic field at Uranus was not known
until Voyager's arrival. The intensity of the field is
roughly comparable to that of Earth's, though it varies much
more from point to point because of its large offset from the
center of the planet. The peculiar orientation of the
magnetic field suggests that the field is generated at an
intermediate depth in the interior where the pressure is high
enough for water to become electrically conducting.

Radiation belts at Uranus were found to be similar in
intensity to those at Saturn. The intensity of radiation
within the belts is such that irradiation would quickly
darken (within 100,000 years) any methane trapped in the icy
surfaces of the inner moons and ring particles. This may
have contributed to the darkened surfaces of the moons and
ring particles, which have lower albedos than coal and are
almost uniform in color.

A high layer of haze was detected around the sunlit pole,
which also was found to radiate large amounts of ultraviolet
light, a phenomenon dubbed 'dayglow'. Surprisingly, the
illuminated and dark poles, and most of the planet, show
nearly the same temperature at the cloud tops.

Voyager found 10 new moons, bringing the total number at
Uranus to 15. Most of the new moons are small, with the
largest measuring about 150 kilometers in diameter.

The five large moons appear to be ice-rock conglomerates like
the satellites of Saturn. Titania is marked by huge fault
systems and canyons indicating some degree of geologic
(probably tectonic) activity in its history. Ariel has the
brightest and possibly youngest surface of all the Uranian
moons and also appears to have undergone geologic activity
that led to many fault valleys and what seem to be extensive
flows of icy material. Little geologic activity has occurred
on Umbriel or Oberon, judging by their old and dark surfaces.

The moon Miranda, innermost of the five large moons, was
revealed to be one of the strangest bodies yet seen in the
solar system. Detailed images from Voyager's flyby of the
moon showed huge fault canyons as deep as 20 kilometers,
terraced layers, and a mixture of old and young surfaces.
One theory holds that Miranda may be a reaggregation of
material from an earlier time when the moon was fractured by
a violent impact.

All nine rings discovered from Earth in the 1970's were
studied by the spacecraft and showed the Uranian rings to be
distinctly different from those at Jupiter and Saturn. The
ring system may be relatively young and did not form at the
same time as Uranus. Particles that make up the rings may be
remnants of a moon that was fractured by a high-velocity
impact or torn up by gravitational effects.

Less was known about Neptune than about Uranus at the
beginning of the Voyager mission. Approximately the same
size as Uranus, Neptune was expected to be a twin except for
having a rotation axis more likely to be normal to the
ecliptic. About five years before the Voyager 2 Neptune
encounter, evidence began accumulating that Neptune had
atmospheric structure and (possibly) rings. The ring data
were very ambiguous; only exotic ring models (transient
rings, partial rings, polar rings, etc.) were consistent
with the observations from Earth.

Even though Neptune receives only three percent as much
sunlight as Jupiter, it is a dynamic planet and showed
several large, dark spots reminiscent of Jupiter's
hurricane-like storms. The largest spot, dubbed the Great
Dark Spot, is about the size of Earth and is similar to the
Great Red Spot on Jupiter. A small, irregularly shaped,
eastward-moving cloud was observed 'scooting' around Neptune
approximately once every 16 hours.

Long bright clouds, similar to cirrus clouds on Earth, were
seen high in Neptune's atmosphere. At low northern
latitudes, Voyager captured images of cloud streaks casting
their shadows on cloud decks below.

The strongest winds on any planet were measured on Neptune.
Most of the winds blow westward, or opposite to the rotation
of the planet. Near the Great Dark Spot, winds blow up to
2,000 kilometers an hour.

The magnetic field of Neptune, like that of Uranus, turned
out to be highly tilted -- 47 degrees from the rotation axis
and offset at least 0.55 radii (about 13,500 kilometers or
8,500 miles) from the physical center. The extreme
orientation may be characteristic of flows in the interiors
of both Uranus and Neptune -- and not related, in the Uranus
case, to the planet's rotation axis tilt or to any possible
field reversals at either planet. Voyager studies of radio
emissions caused by the magnetic field revealed the length
of a Neptunian day (16.11 hours). The spacecraft also
detected auroras, though they are much weaker than those on
Earth and other planets.

Triton, the largest Neptunian moon, was shown to be not only
the most intriguing satellite of the system, but also one
of the most interesting in all the solar system. Intricate
surface patterns suggest a remarkable geologic history,
while Voyager 2 images captured active geyser-like eruptions
spewing invisible nitrogen gas and dark dust particles
several kilometers into the tenuous atmosphere. Triton's
relatively high density and retrograde orbit offer strong
evidence that it is not an original member of Neptune's
family but, rather, is a captured object. If so, tidal
heating could have melted Triton in its originally eccentric
orbit, and the moon may have been liquid for as long as one
billion years after its capture by Neptune.

An extremely thin atmosphere extends about 800 kilometers
above Triton's surface. Nitrogen ice particles may form thin
clouds a few kilometers above the surface. The atmospheric
pressure at the surface is about 14 microbars, 1/70,000th the
surface pressure on Earth. The surface temperature is about
38 K -- the coldest known temperature of any body in the
solar system.

The new moons found at Neptune by Voyager are all small and
remain close to Neptune's equatorial plane.

Searches for 'ring arcs,' or partial rings, showed that
Neptune's rings actually are complete, but are so diffuse and
the material in them so fine that they could not be fully
resolved from Earth. The arcs are confined by the actions of
nearby satellites. Particle sizes are smaller than at Uranus.

The Voyager spacecraft are continuing to return data about
interplanetary space and some of our stellar neighbors near
the edges of the Solar System. Their fields, particles, and
waves instruments are studying the environment around them.
In May 1993, the plasma wave experiment began picking up radio
emissions that originate at the heliopause, the outer edge of
our solar system, where the interstellar medium restricts the
outward flow of the solar wind and confines it within a
magnetic bubble called the heliosphere. By studying the
radio emissions, scientists now theorize the heliopause
exists some 90 to 120 astronomical units from the Sun.

The Voyagers have also become space-based ultraviolet
observatories and their unique location in the universe gives
astronomers the best vantage point they have ever had for
looking at celestial objects that emit ultraviolet radiation.

The cameras on the spacecraft have been turned off and the
ultraviolet instrument is the only experiment on the scan
platform that is still functioning. Voyager scientists
expect to continue to receive data from the ultraviolet
spectrometers at least until the year 2000. At that time,
there will not be enough electrical power for the heaters to
keep the ultraviolet instrument warm enough to operate.

Yet there are several other fields and particle instruments
that can continue to send back data as long as the spacecraft
can stay alive. They include the cosmic ray subsystem, the
low-energy charge particle instrument, the magnetometer, the
plasma subsystem, the plasma wave subsystem and the planetary
radio astronomy instrument. "